The present invention relates generally to phase-locked loops and more specifically to an apparatus and method of making phase-locked loop""s jitter transfer function independent of data transition density.
In order to improve the input/output performance of computer systems and networks, serial interface standards have been proposed and developed. In order to accommodate the serial interface standards, clock recovery circuitry has been developed to recover an embedded clock signal from the incoming data stream. A popular type of clock recovery circuit utilizes a phase-locked loop.
A type of measurement for phase-locked loop performance is the jitter transfer function. The jitter transfer function relates the ratio between the input jitter and output jitter in a specified frequency band. In phase-locked loop clock recovery circuits known to the art, the jitter transfer function is dependent upon the loop filter transfer function and the data transition density. Another factor in the performance of a phase-locked loop is the loop bandwidth. The loop bandwidth is also affected by the data transition density. This makes it difficult to optimize the jitter performance of a phase-locked loop with regard to its noise filtering capability since the loop bandwidth may change with a data pattern change.
The data transition density also affects the loop gain in non-linear phase-locked loops in the passband. This results in the static phase error, a measure of the amount of phase error when there is no noise, being dependent upon transition density. The static phase error affects the jitter performance of a phase-locked loop to a small degree at data rates of a gigabyte per second but greatly affects jitter performance at data rates of five gigabytes per second and higher.
Consequently, it would be advantageous if a method and apparatus existed that allowed a phase-locked loop""s jitter transfer function to be independent of data transition density. It would also be advantageous if a phase-locked loop existed which allowed for an optimization in noise filtering performance by making the loop bandwidth independent of the received data patterns. Further, it would be advantageous if a phase-locked loop existed which allowed for a loop gain in the passband independent of the received data patterns. These advantages may be important as complementary metal oxide semiconductor (CMOS) technology moves into the multi-gigabits per second range and higher.
Accordingly, the present invention is directed to a novel method and apparatus for making a jitter transfer function of a phase-locked loop independent from the data transition density. The present invention is further directed to a phase-locked loop that has a loop bandwidth and a loop gain in the passband which are both independent from the received data patterns.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles of the invention.